Thanks for the kind & prompt response.
What you mentioned is correct; but this kind of provision in IPv6 framework
has resulted in more complex implementations.

Eg., if PPP server would have allocated
one prefix to PPP client (in IPv6CP) then the client would become a globally
routable
host; and the PPP client might use this prefix as per his requirements;
and multiple prefix allotment could also take place in the same way as
defined in IPv6 RFCs!

But since things are not defined in this way, PPP client and
server both have to have DHCPv6 client/server implemented
on them; making both the systems (PPP client & server)
more complex!